Slices of rat, rabbit and guinea pig lung were shown to take up labeled serotonin by an active process that was reminiscent to that observed by other authors in intact lung preparations. The uptake was blocked by Lilly 110140, a compound known to block serotonin transport in neurones and platelets. Since the labeled serotonin taken up by the slice was not released by reserpine as was endogenous serotonin, it was concluded that the label had not entered endogenous stores. Further studies suggested that the uptake and serotonin metabolism were coupled processes. When lung slices were incubated in the presence of an MAO inhibitor, labeled serotonin accumulated within the tissue. In the absence of the inhibitor, serotonin did not accumulate and was rapidly converted to hydroxyindole acetic acid, which diffused back into the medium. In the presence of Lilly 110140, uptake and metabolism were inhibited. Lilly 110140 did not inhibit MAO activity in lung homogenates in concentrations that inhibited uptake and metabolism in the intact slice.